The entry for M Roberts, head girl in 1943, takes up a good chunk of the oak noticeboard recording the honours harvested by generations of old girls, minds expanded and backbones stiffened by an education at Kesteven and Grantham Girls' School.

Now the noticeboard – along with the virtually unaltered Edwardian school holding it – has been given a Grade II listing, partly in honour of its most famous pupil; Margaret Hilda Roberts, later prime minister, later still Baroness Thatcher of Kesteven.

Culture minister Ed Vaizey said: "This is an outstanding example of an Edwardian grammar school with an eclectic architectural character built by the architect HH Dunn, who specialised in education buildings.

"But, as well as its architectural interest, the school has huge historical interest, and the education Margaret Thatcher received there was a formative experience which went on to affect her life and political convictions."

The listing is on the advice of English Heritage, where Emily Gee, head of designation, said: "This handsome Edwardian grammar school possesses real historic interest for its association with Britain's first female prime minister.

"Margaret Thatcher's connection to her home town was a poignant one, and her views on education were no doubt shaped by her experiences in this building"

The listing adds to Grantham's growing Thatcher tourist trail. Five months after her death, fundraising continues for a statue of her, while a plaque marks the corner shop where the daughter of a grocer grew up and formed her political notions of thrift, hard work and balanced books. Among the town museum's proudest possessions are examples of her trademark blue suit and black handbag which she donated.

She first attended Huntingtower Road primary and won a scholarship to the grammar school, now an academy, then still run by its founder, Miss Gladys Williams.

Her name was added to the head girls' board in 1945, two years after she left for Oxford, but it needed repeated updating, eventually taking up twice as much space as most of her peers, recording that she became BA Hons Nat Sc Oxon; BSc; Barrister-at-Law. Much, much else could have been added, but there wasn't room.

The school is immensely proud of its most celebrated old girl. She returned in 1986 to unveil a Wedgwood plaque bearing her portrait in the hall. The school's motto translates as "From this place take true inspiration" and she commented: "It certainly came true for me. From this place I did take inspiration"

The visit also saw her open the Roberts Hall, named not in her honour but for her father, the much-cited alderman Alfred Roberts.